Thermostats of the type used for temperature regulation in liquid-cooling systems are typically disposed in a closed flow loop of conduits which also include a pump and a heat exchanger such as a radiator or similar devices. Such thermostats usually incorporate a valve which may be opened according to the temperature of the cooling liquid. By varying the opening of the valve, control of the flow of liquid to the heat exchanger or radiator can be effected whereby the temperature of the liquid can be regulated. Typically the valve is displaced by a temperature sensitive plunger which acts against a spring which biases the valve to the closed position. In the closed position no liquid flows to the heat exchanger or radiator. However, when the temperature of the liquid exceeds the desired value the plunger displaces the valve into an open position, against the spring, to allow a desired amount of liquid to flow through the heat exchanger or radiator. In this manner the liquid temperature can be controlled.
Failure of the thermostat, for example by the valve being stuck in the closed position, results in a loss of temperature control. In this circumstance the liquid is not directed to the heat exchanger or radiator and as a result the cooling properties of the system are lost. Inevitably the system intended to be cooled becomes damaged due to prolonged overheating. In an automobile engine, for instance, permanent damage can occur when the cooling system has failed.
In automobile thermostats, the temperature sensitive plunger is typically controlled by a thermally expansive composition formed, from among other things, of wax. Thermostats may malfunction for other reasons however, including corrosion to the plunger components. In an extreme overheating situation, the thermostat may be damaged, for example, by the loss of the thermally expansive wax. Regardless of why extreme overheating occurs, thermostats are typically located close enough to the parts being cooled to be damaged by the overheating. However, after extreme overheating occurs the thermostat itself will be damaged resulting in difficulty in determining whether the thermostat was damaged prior to the overheating (and thus was a contributing factor), or whether the thermostat was merely damaged as a result of the overheating. Determining if there was an intrinsic failure of the thermostat is important in apportioning fault, since the cost of repairing or replacing the damaged system, such as an automobile engine, can be very high.
One way to resolve this difficulty is disclosed in U.S. Pat. No. 4,883,225 (Kitchens). This patent discloses providing a thermostat that includes a fusible alloy in a temperature sensitive valve. When the thermostat is subjected to an extreme overheating, the fusible alloy melts causing the valve to be permanently displaced in an open position. The fluid is therefore directed to the heat exchanger even though the thermostat has failed.
The difficulty with this prior approach however is that a thermostat incorporating such a fusible alloy is very difficult to manufacture. In particular, the components of the plunger need to be manufactured under very precise tolerances, which are difficult if not impossible to achieve in mass production. Furthermore, expensive materials are required and additional steps are needed in the manufacturing process in order to assemble this device.